There is currently interest in the use of organic materials for the fabrication of light-emitting diodes. The primary reason is that a large number of organic materials exist with high fluorescence quantum efficiencies in the visible spectrum, and thus have great potential for multi-color display applications.
In organic light-emitting diodes electrons and holes are injected from the contacts into the organic material and form negatively and positively charged polarons. Photon production occurs through the radiative recombination of the oppositely charged polarons. To achieve the best device performance, the energetics of the organic and electronic materials should be matched. ITO is commonly used as the anode because of its high work function of 4.6 eV, thus acting as an effective hole injector. Good device performance will occur when the work function of the cathode is close to the electron affinity of the polymer. Increasing the work function of the cathode reduces the number of injected electrons, resulting in a higher operating voltage or a lower device efficiency.
Several metals are known to have low work functions and thus are ideally suited for electron injection into organic materials. However, they are susceptible to atmospheric oxidation. For instance, Mg has a work function of 3.7 eV and is a good candidate for the electron injector, while Tang and VanSlyke in Appl. Phys. Lett. Vol. 51, 1987, p. 913-915 described a stability test in which light-emitting diodes with alloyed Mg-Ag cathodes show a steady degradation accompanied by an increase of the drive voltage. Some of the failure is attributed to the degradation of the contacts. To improve the stability a thick indium film greater than one micron is used for encapsulation, as described in U.S. Pat. No. 5,073,446.
One of the major shortcomings presented by the conventional organic light-emitting diode structure is the difficulty in achieving monolithic integration, where, for example, an array of organic light-emitting diodes and the driver electronics are fabricated on a single chip.
In a normal configuration, the light-emitting diode consists of an electron-injecting metal contact on the front surface of an electroluminescent layer on a conductive glass substrate. When a semiconductor wafer such as Si is used as a substrate, the light emission through the substrate is prohibited because of the opaqueness of the substrate. Therefore, a transmissive top electrode is necessary. Since the materials used for the electron injector are highly reactive with oxygen, a thick cathode layer completely encased in several microns of more inert metals such as indium or gold is commonly used, thus light emission through such a top surface is blocked. With a reverse structure where the electron injector is in contact with Si and the hole injector is on top of organic layers, light emission through the top electrode is possible because a transparent indium-tin oxide layer or a semi-transparent thin gold layer can be used as the anode. However the low work function metals are either highly reactive with Si and/or act as a fast diffusing species, thus significantly affecting the device performance.
It is therefore highly desirable to provide a material which has a low work function and yet is relatively stable against oxidation as the cathode for the organic light-emitting diodes. It is also highly desirable to provide a low work function material which is compatible with some semiconductors such as Si, thus allowing integrating organic light-emitting diodes and electronic devices on the same chip.